1. Field of the Invention
This invention relates to an optical-fiber light amplifier which is to be used in a light communication system and uses an optical fiber doped of a rare earth element.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows a conventional optical-fiber light amplifier of the type described above, the amplifier being to be used for a two-system signal line. In FIG. 1, reference numerals 1a, 1b designate first and second rare-earth-doped optical fibers; 2a, 2b, first and second excitation light sources; 3a, 3b, first and second multiwavelength combining and dividing devices; 4a, 4b, first and second excitation light source drive circuits; 5a, 5b, 5c, 5d, input and output terminals for signal light; and 11a, 11b, first and second isolators.
In operation, each of the first and second rare-earth-doped optical fibers 1a, 1b is a single-mode optical fiber doped of a rare earth element such as erbium (Er) and having a length of several meters to several tens meters. The first multiwavelength combining and dividing device 3a is connected to the first rare-earth-doped optical fiber 1a. The multiwavelength combining and dividing devices may be optical couplers. The first and second excitation light source 2a, 2b are semiconductor lasers having a wavelength of, for example, 1.48 .mu.m and are driven the first and second excitation light source drive circuits 4a, 4b, respectively. When several mW to several tens mW of first excitation light outputted from the first excitation light source 2a is inputted to the first rare-earth-doped optical fiber 1a via the first multiwavelength combining and dividing device 3a, the first rare-earth-doped optical fiber 1a assumes an inverted distribution state so that the signal light having a wavelength of 1.53 or 1.55 .mu.m and inputted from the input and output terminal 5a for the signal light is amplified by the action of induced emission for output to the input and output terminal 5b. Likewise, when the second excitation light outputted from the second excitation light source 2b is inputted to the second rare-earth-doped optical fiber 1b via the second multiwavelength combining and dividing device 3b, the second rare-earth-doped optical fiber 1b assumes an inverted distribution state so that the signal light inputted from the input and output terminal 5c amplified for output to the input and output terminal 5d. With this conventional arrangement, it is impossible to improve the reliability of this type optical-fiber light amplifier.
The simplest popular optical-fiber light amplifier for a single-system signal has a construction such as shown in FIG. 2. This known art is exemplified by Japanese Patent Laid-Open Publication No. Hei 2-241073. In this light amplifier, two sources for excitation light are combined by a combining device. In FIG. 2, reference numeral 1 designates a rare-earth-doped optical fiber; 2a, 2b, first and second excitation light sources; 3, a multiwavelength combining device for combining excitation light and signal light; and 8, a combining device for combining the first excitation light and the second excitation light. However, when the first excitation light and the second excitation light are combined by the combining device 8, the combined light power will be 1/2 of the total light power of the first and second excitation light emitted from the first and second sources 2a, 2b. Further, since the first excitation light and the second excitation light interfere with one another, the output power of the combining device tends to fluctuate and hence to be non-stable.
FIG. 3 shows a conventional optical amplifier for a double-system signal, which is a natural expansion of the construction of FIG. 2. First excitation light and second excitation light outputted from the first and second sources 2a, 2b are combined and then divided by a combining and dividing device 8, and the resulting separate parts of excitation light are inputted to first and second rare-earth-doped optical fibers 1a, 1b via first and second multiwavelength combining devices 3a, 3b. However, the output of the combining device 8 is non-stable so that the amplifying characteristic of the rare-earth-doped optical fibers 1a, 1b will not be stable.
With this conventional arrangement, the level of excitation light to be inputted to the amplifying media will not be stable.
FIG. 4 shows another conventional light amplifier, which is disclosed in "The Impact That An Er-doped Optical-Fiber Amplifier Contributes To Light Communication" by T. Shimada, Oplus, No. 113, pp. 75-82, 1989. In FIG. 4, reference numeral 1 designates a rare-earth-doped optical fiber; 2, an excitation light source; 3, a combining and dividing device; 4, an excitation light source drive circuit; and 5, an optical fiber serving as a signal transmission path.
In operation, the rare-earth-doped optical fiber 1 is a single-mode optical fiber doped of a rare earth element such as erbium (Er) and having a length of several meters to several tens meters. The combining and dividing device 3a is connected to the rare-earth-doped optical fiber 1. The combining and dividing device may be an optical coupler. The excitation light source 2 is a semiconductor laser having a wavelength of, for example, 1.48 .mu.m and is driven the excitation light source drive circuit 4. When several mW to several tens mW of excitation light outputted from the excitation light source 2 is inputted to the rare-earth-doped optical fiber 1 via the combining and dividing device 3, the rare-earth-doped optical fiber 1 assumes an inverted distribution state so that the signal light having a wavelength of 1.53 or 1.55 .mu.m and inputted from the optical fiber 5 is amplified by the action of induced emission. The rare-earth-doped optical fiber 1, the excitation light source 2, the combining and dividing device 3 and the excitation light source drive circuit 4 jointly constitute a light amplifying means.
In the light amplifier utilizing the action of induced emission, natural emission light is emitted along with the action of light amplification. This natural emission light produces noise, which deteriorates the signal-to-noise rate of signal light. The noise resulting from natural emission light is exemplified by shot noise due to natural emission light, beat noise between signal light and natural emission light, and beat noise between one natural emission light and another natural emission light. Natural emission light is free of polarization dependency and has the component parallel to the polarization plane of signal light and that perpendicular thereto.
Further, when two or more light amplifiers are to be used as connected in tandem, accumulation of natural emission light brings down the saturation level of signal gain of the light amplifier.
With the conventional light amplifier, the noise resulting from natural emission light would deteriorate the signal-to-noise ratio of signal light. If two or more of the conventional light amplifiers are used as connected in tandem, accumulation of natural emission light would bring down the saturation level of signal gain of the individual light amplifier.
FIG. 5 shows a convention light amplification repeating system, which is disclosed in Japanese Patent Laid-Open Publication No. Hei 3-214936.
In this conventional light amplification repeating system, when the main signal light c, on which a subcarrier a is superposed, is inputted from a main signal light input terminal 1, the main signal light c is branched by a light branching coupler 14. Part of the branched main signal light c is inputted to a subcarrier processing circuit 20 where a process takes place based on the subcarrier a. A subcarrier generator 15 generates a subcarrier b based on the subcarrier a, and an excitation light source drive circuit 18 drives an excitation light source 19, and a rare-earth-doped optical fiber 12 modulates, in intensity, main signal light c, whereupon main signal light d, on which the subcarrier b is superposed, is transmitted to the repeating system at the subsequent stage. At that time the subcarrier b to be superposed has a frequency different from the subcarrier a.
With this arrangement, the subcarrier to be processed by the subcarrier processing circuit must have a different frequency for every light repeating system.
FIG. 6 shows a typical light amplification repeating system equipped with light amplifiers and utilizing light excitation. The technology of this kind is disclosed in detail in, for example, Japanese Patent Laid-Open Publication No. Hei 3-214936. In FIG. 6, reference numerals 100a, 100b designate light end offices; 101c, 101d, 101e, light amplification repeaters; 102b, 104a light-to-electric converters; 103a, 103b, subcarrier terminals; 102a, 104b, electric-to-light converters; 105c, 105d, 105e, 107c, 107d, 107e, light amplifiers; and 106c, 106d, 106e, subcarrier transmitters.
In the conventional supervisory method, assuming that an abnormality, such as a fiber breakage between the light amplifiers 105c, 105d or a fault of the light amplifier occurs, the subcarrier transmitter 106c generates a subcarrier to modulate the light amplifier 105d, which is an excitation LD, and transmits the subcarrier to the downstream side. The subcarrier terminal 103b discriminates the context of the transmitted subcarrier and issues an appropriate command.
FIG. 7 shows the conventional light amplifier and subcarrier transmitter which are shown in Japanese Patent Laid-Open Publication No. Hei 3-214936. In FIG. 7, reference numeral 105 designates a light amplifier; 106, a subcarrier transmitter; 111, an erbium-doped fiber; 112a, 112b, light isolators; 113, a light branching device; 114, a light combining device; 115, an excitation LD; 116, a subcarrier processing circuit; 117, a subcarrier generator; and 118, an excitation LD drive circuit. FIGS. 8A, 8B and 8C show the signal and spectrums of the system of FIG. 7.
In operation, the signal light is inputted to the light branching device 113 via the isolator 112a. The light branching device 113 branches a small quantity of the power to the subcarrier processing circuit 116 and most of the remaining power to the erbium-doped fiber 111. In the subcarrier processing circuit 116, the state of light input to the light amplifier is detected. In the subcarrier generator 117, a subcarrier is generated based on information from the subcarrier processing circuit 116 and the state of operation of the light amplifier. The information from the subcarrier processing circuit is a breakage of input to the light amplifier when, for example, the upstream fiber happened to be cut off. What to be supervised while the light amplifier is operative may be a fault of the excitation LD. The excitation LD drive circuit drives the excitation LD by a current obtained by superposing the subcarrier on a bias current. The excitation light is modulated in intensity by the subcarrier.
FIG. 8A schematically shows the output light from the excitation LD 115 to the light combining device 114 in FIG. 7; a signal slightly modulated by 1 to several % is superposed on a direct current.
Since the gain of the light amplifier is very dependent on the excitation light power, it will be modulated as the excitation light is modulated in intensity. Consequently the envelope of signal light is modulated in intensity by the subcarrier.
The transmission characteristic of the light amplification repeating and transmission system is a band pass characteristic; it has therefore been customary to premodulate the subcarrier by a sine wave having predetermined binary information. This concept is discussed in detail in, for example, Japanese Patent Laid-Open Publication No. Hei 3-252231 and "Control of Supervision in Light Amplification Repeating and Tranmission Method" by Imai et al., Electronic Information Communication Society Spring Meeting B-944, 1992.
Transmission of a subcarrier requires a high transmission quality. On many occasions the quality of signal is evaluated on the carrier/noise ratio (C/N ratio). The present inventors discovered that the main factor to deteriorate the C/N ratio of a subcarrier is the intensity noise produced from the light amplifier. The intensity noise is expressed quantitatively by relative intensity noise (RIN).
FIG. 8B shows a frequency spectrum of electric power density of the output light from the excitation LD 115 in FIG. 7. As shown in FIG. 8B, in the conventional excitation in which a direct current is modulated by a subcarrier, noise would occur chiefly at the low-frequency side so that the output of the light amplifier 112b of FIG. 7 will be as indicated by the frequency spectrum of FIG. 8C. Consequently the frequency of the subcarrier deteriorates the C/N ratio.
FIG. 9 shows the result of calculation of the C/N ratio when the modulation degrees of the subcarrier is varied. When RIN is -$, the C/N ratio is determined by the natural emission light of the light amplifier; thus 90 dB is obtained with a modulation degree of 1%. However, the C/N ratio varies markably, compared to RIN. In fact, the measurement of RIN of the output light from the light amplifier shows that RIN at 5 kHz was -82 dB/Hz. Calculating the C/N ratio using this value, the C/N ratio at a modulation degree of, for example, 1%, was only 20 dB. Consequently an adequate code error cannot be achieved.
The mode of supervising operation for any breakage of communication channel in the foregoing circuit will now be discussed.
Assuming that the fiber between the light amplifiers 105c, 105d is cut off, "input is cut off" will be judged, based on the fact that the light power branched from the branching device 113 in the subcarrier transmitter 106d of the light amplification repeater 101d is lowered from a threshold preset in the subcarrier processing circuit 116. Instantly the subcarrier generator 117 generates a subcarrier, which will modulate the excitation LD of the light amplifier 105d and will then be transmitted to the downstream side.
Also in the light amplification repeater 101e at the next stage, the light power will be lowered to start generating a subcarrier, but the light power outputted from the upstream light amplifier 105d is increased progressively (the time constant at this time depends on the relaxation time of natural emission from the light amplifier) so that the subcarrier processing circuit in the light amplification repeater 101e at the next stage will stop transmission of the carrier signal soon.
With this light amplification repeater, the C/N ratio of a supervisory signal would be lowered to deteriorate the quality of the supervisory signal.
When detecting any breakage of signal channel, the subcarrier terminal 103b will receive a subcarrier, indicating the fact "input is cut off", not only from the light amplification repeater 101c but also the light amplification repeater 101d. Consequently it was impossible to discriminate as to which information is correct.